The bars were stacked casually on a desk in a back office of Dubai's bustling gold souk, and they immediately caught the eye of the inspectors. Keen to impress his guests, the manager picked one up.

After a lifetime in the trade, Osama Kaloti suggested he could tell the provenance and purity of each bar simply by its look and weight. This one was silver.

Or so it seemed. Kaloti insisted the metal in his hand was gold. To prove his point, he scraped away the outer coating, revealing the yellow treasure beneath. There were about six bars in the stack, which might have weighed about 75kg in total. If gold, at today's prices, they would be worth $3.2m. If silver, the same size bars would be valued at less than $30,000.

The inspectors, from Ernst & Young Dubai, had been hired to audit Osama's family business Kaloti Group, Dubai's largest gold refinery, a process designed to assure important international customers that the company was doing all in its power to avoid conflict gold.

Processing some 300 tonnes a year, Kaloti claims to account for close to half of all refining in Dubai's gold market. Regulators claim gold trading in the emirate reached $70bn in 2012, up 25% on the previous year.

"Dubai is emerging as a gold bullion centre to rival London, Shanghai and others," Osama's father, Munir Kaloti, who founded the business, explained to the Middle East Economic Digest in April last year. "Dubai now accounts for approximately 25% of the world's annual gold trade. The principal challenge [our] company faces is ensuring the responsible sourcing of gold … Our safeguard measures are beyond the minimum set out by trade associations and government bodies."

But back in the souk office, only the previous month, E&Y inspectors could hardly believe what Osama was telling them. According to their minutes: "He took a scanner and showed that the gold content was more than 85% and these bars are … from a Moroccan supplier. He said that it's normal to receive silver coated gold bars especially from Morocco due to the gold export limits imposed by the Moroccan customs."It also emerged the Moroccan suppliers had brought the shipment in by hand and were paid in cash at Kaloti's office in the souk. The shipment had left North Africa incorrectly labelled silver, and was properly accepted at Dubai airport customs as gold. It sounded like a smuggling scam.

Further investigations found about four tonnes of gold, hidden beneath silver plating, may have come to Kaloti in a similar manner from several suppliers in Morocco. At today's prices, which are lower than those of 2012, four tonnes of gold is worth more than $170m.

After being melted down, the bars would eventually emerge from the refinery as newly minted bullion, of 99.5% purity or greater, destined for the international gold market and stamped with a serial number and the words "Kaloti, Dubai".

Following the E&Y inspection, Kaloti expected to be able to claim that its gold bars had been independently verified as conflict-free and responsibly sourced. But the inspectors were rapidly taking a very different view. They were shocked by refinery admissions that it accepted gold which had probably been smuggled. This was to be just one of the failings they stumbled over but it was enough, they thought, to mean the refinery had already effectively failed its inspection and would receive from E&Y the worst available score for the review period, covering the last seven months of 2012: "Breach of review protocol and zero tolerance".